Steve Jobs will take leave of absence from Apple until the end of June for health reasons.
He said in a letter to employees Wednesday that "my health-related issues are more complex than I originally thought."
"In order to take myself out of the limelight and focus on my health, and to allow everyone at Apple to focus on delivering extraordinary products, I have decided to take a medical leave of absence until the end of June," Jobs wrote.
He said "curiosity" over his health "continues to be a distraction" for himself and for the company.
Apple's stock was down 6.21% to $80.03 in after-hours trading on the news, having fallen 2.71% during the day.
COO Tim Cook will take over running day-to-day operations, but Jobs said he would remain involved in major strategic decisions.
His decision to take medical leave follows months of speculation about his health over his visible weight loss.
In a letter last week he attributed that to a "hormone imbalance", which would keep him from delivering his customary keynote address to Macworld.
But he said in that letter that the remedy was "simple and straightforward".
Ryan Jacob, of the Los Angeles-based Jacob Internet Fund, which owns a stake in Apple, told the New York Times the letter indicated that "his health situation is probably more serious than we thought,"
"It's disturbing. I think it's going to be especially disturbing to people given the letter of last week," Jacob said.